An in-line degassing operation is usually done by insufflation of an appropriate inert gas containing some percentage of Chlorine (Cl2) gas. The Chlorine gas forms as small bubbles in the molten metal. The degassing is generally done in a continuous operation just before the casting, which may itself be done continuously. A mixture of inert gas and Cl2 (Chlorine) is injected into the molten metal to treat the molten metal as it flows from the furnace to the casting pit. While inert gas alone can effectively remove dissolved H2 (hydrogen) through mass transfer, removing alkali and alkaline earth impurities (such as sodium (Na), lithium (Li), and calcium (Ca)) in the molten metal requires a chemical reactant such as Cl2, as given by the following reactions:2Na+Cl2→2NaCl andCa+Cl2 →CaCl2 
Chlorine (Cl2) may also improve the floatation and removal of non-metallic inclusions, providing improved metal cleanliness.
However, the use of gaseous Cl2 represents an environmental and industrial hygiene issue. Gaseous Chlorine is also a source of regulated air emissions. Furthermore, because of the hazardous nature of Cl2, the storage, piping, safety, and training requirements can also be stringent. Also, Cl2 can cause increased corrosion and wear of other equipment in a plant. Thus, it may be desirable to remove alkali and alkaline earth metals from molten aluminum and its alloys in-line without the use of Cl2.
To achieve effective degassing, all degassing apparatus must deliver a certain minimum volume of gas per kilogram of metal. Degassing can be performed in a trough-like or a deep box degasser. A trough-like degasser is a degasser with a static volume/dynamic volume ratio less than at least 50% of a deep box degasser static volume/dynamic volume ratio and one which retains little if any metal when the source of metal is interrupted after the degassing operation is completed. In a trough-like degasser where the residence time of the metal in the region in which, the gas is supplied is substantially less than in the deep box degassers, the amount of gas which each rotary injector must deliver is high and the ability to deliver a suitable amount of gas determines the effectiveness of an injector design.
It has been noticed that in a trough-like degasser with gas rotors capable of delivering a suitable volume of gas to a molten metal that gases tends to be released from the rotors in an irregular manner causes splashing at the surface of the molten metal and inefficiency of dissolved gas removal. Some trough-like degassers use several relatively small rotary gas injectors along the length of a trough section to achieve the equivalent of a continuous or pseudo “plug” flow reactor rather than a well-mixed flow reactor or continuous stirred-tank reactor (CSTR), which is characteristic of deep box degassers. In an ideal plug-flow reactor there is no mixing and the fluid elements leave in the same order they arrived. Therefore, fluid entering the reactor at time t will exit the reactor at time t+τ, where τ is the residence time of the reactor (E(t)=δ(t−τ)). An ideal continuous stirred-tank reactor is based on the assumption that the flow at the inlet is completely and instantly mixed into the bulk of the reactor. The CSTR and the outlet fluid have identical, homogeneous compositions at all times. An ideal CSTR has an exponential residence time distribution ((E(t)=(1/τ)e(−t/τ)).
However, trough-like degassers with a plurality of small rotary gas injectors are not capable of delivering large volumes of gas in the form of fine bubbles into molten metal without substantial irregularities of gas flow and are not suitable for use in any application in which such high gas delivery in the form of fine bubbles is required. FIGS. 1A and 1B illustrate that a deep box degasser (such as Alcoa A622) is more efficient to remove Hydrogen and inclusions from molten metal than a trough-like degasser (such as ACD) when chlorine is used as a degassing agent. The same results are expected when chlorine is replaced by a salt flux mixture. Therefore, deep box degassers must be utilized to reduce splashing at the surface of the molten metal and to maximize the efficiency of dissolved gas and inclusion removal.